Photographs, paintings, film material and other artistic works have for many years been recorded and transmitted using analogue carriers. However, their reproduction and processing is time consuming, involves a heavy workload and leads to degradation of the original material. This means that content produced and stored using analogue devices has an in-built protection against unintentional changes and malicious manipulation. In general, deliberate changes in analogue media are not only difficult but can easily be perceived by a human inspector.
Recently however, digital media have become pervasive, and threaten to completely substitute their analogue counterparts. Furthermore, affordable media processing tools and fast transmission mechanisms are ubiquitous. As a consequence digital content can nowadays be accurately copied, processed and distributed around the world within seconds. Creators, legitimate distributors and end-users enjoy the flexibility and user friendliness of digital processing tools and networks to copy, process and distribute their content over open digital channels at high speed. However, they also need to guarantee that material used or being published at the end of the distribution chain is genuine. Consequently, automatic tools to establish the authenticity and integrity of digital media are highly important.
Secure communications problems have largely found a solution in cryptography, which guarantees message integrity by using digital signatures with secret keys. However, traditional cryptosystems do not permanently associate cryptographic information with the content. Cryptographic techniques do not embed information directly into the message itself, but rather hide a message during communication.
To provide security by using signatures embedded directly in the content, additional methods need to be considered. Techniques that have been proposed to address this problem belong to a more general class of methods known as digital watermarking, as for example may be found in Signal Processing, Special Issue on Watermarking, vol. 66, no. 3 May 1998.
Several watermarking schemes that address image authentication have been previously developed and fall into two basic categories: fragile and semi-fragile.
Fragile watermarking schemes address the detection of any image changes. Semi-fragile watermarking schemes are designed to discriminate between expected image changes, in most cases due to application constraints, e.g., compression to meet bandwidth requirements, and intentional image tampering.
In the case of fragile watermarking, a number of schemes exist in the prior art:
One prior scheme is proposed in S. Walton, “Information Authentication for a Slippery New Age”, Dr. Dobbs Journal, vol. 20, no. 4, April 1995, pp. 18-26. The scheme uses a check-sum built from the 7 most significant bits of a given pixel, which is then inserted as the least significant bit of the pixel. However, the watermark has only limited security, primarily due to the ease of calculating new check-sums.
Another prior fragile watermarking scheme is proposed in M. M. Yeung and F. Mintzer, “An Invisible Watermarking Technique for Image Verification”, Proc. ICIP, Santa Barbara, Calif., 1997. The Yeung-Mintzer algorithm uses a secret key to generate a unique mapping that randomly assigns a binary value to grey levels of the image. This mapping is used to insert a binary logo or signature in the pixel values. Image integrity is inspected by direct comparison between the inserted logo or signature and the decoded binary image. The main advantage of this algorithm is its high localization accuracy derived from the fact that each pixel is individually watermarked. However, the Yeung-Mintzer algorithm is vulnerable to simple attacks as shown in J. Fridrich, “Security of Fragile Authentication Watermarks with localization”, Proc. SPIE, vol. 4675, No. 75, January 2002.
A third prior scheme for image authentication is proposed in P. W. Wong, “A Public Key Watermark for Image Verification and Authentication”, Proc. ICIP, Chicago, Ill., October 1998. This scheme embeds a digital signature extracted from the most significant bits of a block of the image into the least significant bit of the pixels in the same block. However this scheme was shown to be vulnerable to a counterfeiting attack in M. Holliman and N. Memon, “Counterfeiting Attacks on Oblivious Block-Wise Independent Invisible Watermarking Schemes”, Proc. IEEE Trans. on Image Processing, vol 9, no 3, March 2000, pp. 432-441. This attack belongs to the class of vector quantization counterfeiting and has been shown to defeat any fragile watermarking scheme that achieves localization accuracy by watermarking small independent image blocks.
One common feature of these and other prior schemes from the literature is that authentication signatures are embedded in the image content, either in the pixel or a transform domain, and the security of the schemes resides in a hash or encryption mechanism.
This ultimately leaves such schemes vulnerable to the attacks noted above.
Thus there is a need for an alternative method of fragile watermarking.
The purpose of the present invention is to address the above problem.